The desulphurization facility at generating unit four is connected to the unit four boiler which uses lignite coal as fuel.
The flue gasses that exit the boiler contain SO2, CO2,
NOx, dust, ash and other materials. Two draught fans draw the flue
gasses from the boiler via two channels to the electrostatic filters
where more than 99.8 percent of the ash is deposited. The flue gasses
cleared of the particles can be directed in two directions by the
draught fans. The normal route is through the desulphurization system
but in emergencies, they can be conveyed directly into the chimneystack
via the bypass hatches.
During normal operation, the flue gasses are passed through a heater
named GAVO, while the bypass hatches are closed. The GAVO is a
regenerative rotating flue gas heater in which the unfiltered flue
gasses with a temperature of 160 °C transfer their heat to the heating
banks, and these then return the heat to the filtered flue gasses,
before entry into the chimneystack to keep their temperature above dew
point (90-95 °C).
The generating unit four desulphurization facility also receives,
when possible, flue gasses from units 1-3 which do not have a dedicated
desulphurization facility.
Unfiltered flue gasses from the flue gas
heater enter the scrubber at a temperature of 120 °C, where they are
sprayed with a suspension of the absorption material (ground limestone;
CaCO3 + water). The part of the scrubber above the level of
the suspension has absorption material spray nozzles fitted in six
levels. Each system of nozzles has an adjoining pump, which pumps the
suspension from the scrubber’s collector, and the nozzles guarantee a
fine spray. The cooling of the flue gasses and an essential phase in the
filtration process, the diffusion of the sulphur dioxide, fluorides and
chlorides through the bordering gas strata onto the surface of the wet
particle of the absorbent occurs in the area of the spray nozzles.
The flue gasses moisten to a point of saturation and cool down to 60
°C in the scrubber. Because they contain finely sprayed droplets, they
must travel through the droplet separator at the top of the scrubber
where the droplets are separated along with other crystallization cores.
The gasses partially vaporize the suspension when passing through the
scrubber, and the vapours exit the system. The steam lost in this
process is replaced by additional water, with which we wash the
separator and the processing water reservoirs. An induced draught fan
that is built into the flue gas channel raises the pressure of the
filtered flue gasses that exit the droplet separator. The fan propels
the filtered gasses from the scrubber over the flue gas heater and the
hatches to the chimneystack. The calcium sulphite, which is produced in
the spray area of the scrubber, falls into the collector. There it has
enough time to oxidise into calcium sulphate. There are three blowers
for the oxidation process, which supply the necessary quantity of air
into the suspension with the calcium sulphite, which oxidizes into
calcium sulphate because of the mixing and the intense influx of air.
For an efficient process of elimination of SO2 from the flue gasses, it is very important to maintain a pH between 5.7 and 6.0, along with the right density and temperature.
The ground limestone, which is used as the absorption material in the
desulphurization process, is obtained from the local silos, under which
two reservoirs for the preparation of the limestone suspension are
located. The water needed for the suspension flows from the circulation
water reservoir.
The limestone suspension, which turns into gypsum in the scrubber, is
pumped into the drainage station where it is condensed on the
hydro-cyclone. The condensed gypsum is conveyed to a produce reservoir
and pumped through a circular pipe system into the produce mixer where
it is mixed with ash. The ash is pneumatically transported from below
the electrostatic filter into the 2,000 cubic meter ash silo. The ash
and the gypsum are dosed in the mixer and after the mixing, we have a
final produce with stable physical and chemical properties. This produce
is then conveyed to a temporary dump where it is loaded onto transport
vehicles and driven to final repository locations.